Effects of Mulch Cover Rate on Interrill Erosion Processes and the Size Selectivity of Eroded Sediment on Steep Slopes

Mulching with vegetative residue is an effective soil conservation practice. A better understanding of sediment characteristics associated with various mulch rates would improve the use of this practice for soil conservation. An experiment was conducted to evaluate the effects of straw mulch on runoff, erosion, and the particle-size distribution (PSD) of eroded sediment. Straw mulch rates of 0, 15, 30, 50, 70, and 90% cover were tested using simulated rainfall. The effective PSD of sediment (undispersed) was compared with equivalent measurements of the same samples after dispersion (ultimate PSD) to investigate the detachment and transport mechanisms involved in sediment mobilization. The maximum stream occurred at a different time from the peak sediment concentration during rainstorms under low mulch rates, which indicated the predominance of supply-limited conditions. However, at higher mulch rates the erosion processes were typical of a transport-limited sediment regime. The ratio of the sediment transported as primary clay to the soil matrix clay content was always less than 1, meaning that most of the clay was eroded in the form of aggregates. Transport selectivity was reflected by the silt enrichment, and silt-sized particles were transported mainly as primary particles since their effective-ultimate ratio was close to 1. The enrichment ratios for the sand-sized fractions decreased from 0.98 to 0.38 with increased mulch rates, and effective-ultimate ratios for sand-sized particles were always greater than 1, indicating that most of these particles were predominantly aggregates of finer particles, especially at high mulch rates. The findings reported in this study have important implications for the assessment and modeling of interrill erosion processes.